The subject invention pertains generally to AM modulation and specifically to a method and apparatus for determining the modulation factor for an AM signal wherein the modulation signal is an A. C. wave.
The amount of AM modulation imparted to an A. C. carrier is reflected by the AM modulation factor, viz. the ratio of incremental change in peak amplitude imparted to the A. C. carrier by virtue of the modulation signal to the unmodulated A. C. carrier peak amplitude. In order to avoid distortions produced by exceeding 100% AM modulation, prior to radio transmissions the AM modulation factor is normally set by the operator on an indicting meter to some predetermined level (usually 90%) by varying the output level of the audio compressor in the radio transmitter while modulating the A. C. carrier with a sinusoidal signal. Since the indicating meter which reads the modulation factor is designed to be accurate for only one level of transmitted RF power corresponding to a predetermined A. C. carrier amplitude, any deviation in transmitted RF power therefrom inherently results in an erroneous reading which can become quite significant with increasing power deviations. Although some radio transmitters are equipped with special circuitry for maintaining a constant level of transmitted RF power irrespective of carrier frequency which obviates the foregoing problem, many common transmitters do not. These latter transmitters may have substantial RF power variations over their operating carrier frequency ranges (in some cases the highest power level being twice that of the lowest power level) which necessitates that the operator apply a correction factor obtained from a chart to the modulation factor meter reading which varies with the meter reading in order to obtain a reliable indication of percent modulation. This, of course, is a nuisance and imposes a burden on the operator.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved method and apparatus for determining the modulation factor for an AM signal wherein the modulation signal is an A. C. wave.
It is a further object of the present invention to provide such a new and improved method and apparatus for determining the AM modulation factor which is accurate irrespective of the A. C. carrier level.
It is still a further object of the present invention to provide such a new and improved method for determining the AM modulation factor which may be implemented through apparatus having a simple and inexpensive design so as to be readily competitive with prior art indicators which are accurate for only one R. F. power level and therefore require correction by the operator at other power levels.
These objects as well as others and the means by which they are achieved through the present invention may best be appreciated by referring to the Detailed Description of the Preferred Embodiment which follows hereinafter together with the appended drawings.